Odyssey in Polyphasic Catalysis by Metal ... - Wiley Online Library

Personal Account DOI: 10.1002/tcr.201600050

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Odyssey in Polyphasic Catalysis by Metal Nanoparticles Audrey Denicourt-Nowicki*[a] and Alain Roucoux*[a]

Chem. Rec. 2016, 16, 2127–2141

C 2016 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim V

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ABSTRACT: Nanometer-sized metal particles constitute an unavoidable family of catalysts, combining the advantages of molecular complexes in regards to their catalytic performances and the ones of heterogeneous systems in terms of easy recycling. As part of this research, our group aims at designing well-defined metal nanoparticles based-catalysts, in non-conventional media (ionic liquids or water), for various catalytic applications (hydrogenation, dehalogenation, carboncarbon coupling, asymmetric catalysis) in mild reaction conditions. In the drive towards a more eco-responsible chemistry, the main focuses rely on the search of highly active and selective nanocatalysts, in association with an efficient recycling mainly under pure biphasic liquid-liquid conditions. In this Personal Account, we proposed our almost fifteen-years odyssey in the world of metal nanoparticles for a sustainable catalysis. Keywords: ionic liquids, nanoparticles, polyphasic catalysis, sustainable chemistry, water

1. Introduction In the two last decades, nanometer-size metal particles have been intensively pursued as advanced catalytic species, owing to their original properties, such as a large surface area-tovolume ratio, giving rise to numerous active sites and thus affording original surface reactivities.[1–3] These nanospecies are generally considered to be at the border between molecular complexes, which constitute a pool of easily tuned, highly active and selective catalysts, and heterogeneous systems, that allow easy recycling and separation from the reaction mixture, as well as a potential use in continuous flow processes (Figure 1).[4–5] Moreover, in some applications, nanocatalysts have also proved to afford novel or divergent reactivity and/or selectivity, compared to their molecular or heterogeneous counterparts.[6–7] Various synthetic methods have been developed to design metal nanoparticles, possessing well-defined sizes, morphologies and compositions, thus enhancing their catalytic performances and selectivity by tailoring their structural and chemical properties at the atomic scale.[8–12] Close to pure homogeneous species in terms of behavior,[13] soluble metal nanoparticles or colloids have received a considerable amount of attention from several research groups, and particularly from experts in homogeneous catalysis. The strategy adopted for their synthesis usually relies on the reaction media (“aqueous “vs. “organic”), as well as on the type of capping agents used.[14] Based on our previous expertise in the synthesis of water-soluble homogeneous complexes for biphasic liquidliquid catalysis, our group has published in 1999 the

stabilization of metallic nanospecies in water,[15] a research area still rather underexplored (Figure 2). Starting from this pioneering example, our Nanogroup has developed a library of easily synthesized hydroxyalkylammonium salts as stabilizing agents of metal nanoparticles in water. Among the various synthetic approaches, the aqueous suspension of active nanospecies was obtained by chemically reducing the water-soluble metal salt in dilute aqueous solution of protective agents. The first catalytic investigations have dealt with the challenging hydrogenation of arene derivatives, in neat water and under very mild conditions (1 bar H2, room temperature), then followed by many other applications, in dehalogenation and carbon-carbon coupling reactions, in asymmetric catalysis up to the recent C-H activation through oxidation processes (Figure 3). The obtained surfactantcapped particles in water provide relevant multiphase catalytic systems, thus affording a convenient strategy for easy recycling and reuse through a biphasic approach.[16] Moreover, these soluble metal nanoparticles were used for their heterogenization on inorganic supports, such as SiO2, TiO2, or mesoporous materials, via a wet impregnation methodology (Figure 3). Finally, the synthetic strategy was easily adapted to ionic liquids, which could be used as reaction media and capping agents for the metallic species. However, to improve the lack of enduring stability during catalytic processes, N-donor ligands were used as extra protective agents of metal nanoparticles in nonaqueous ionic liquids. Finally, in both reaction media (water or ionic liquids), all colloidal suspensions were investigated in various polyphasic catalytic processes, thus affording easy separation and recyclability of the dispersed metal nanoparticles.[17–18]

[a]

A. Denicourt-Nowicki, A. Roucoux ENSCR, UMR, CNRS 6226 11 Allee de Beaulieu, CS 50837 35708 Rennes Cedex 7 (France) E-mail: [email protected]; [email protected]

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2. Metal NPs Dispersed in Ionic Liquids for Biphasic Catalysis In the last decade, ionic liquids (ILs) have emerged as relevant reaction media for applications in polyphasic catalysis,



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Fig. 1. Nanocatalysis: a bridge between homogeneous complexes and heterogeneous catalysts.

allowing easy product separation and catalyst recovery,[19–22] and have been intensively pursued as an adequate medium for the preparation of “soluble” transition metal nanoparticles.[23–26] Two main strategies are described in the literature. In a first approach, in addition to being the reaction media, ionic liquids could also provide an electrostatic stabilization for metal nanospecies, thus playing the role of a “nanosynthetic template”.[27–28] In that case, no extra stabilizing molecules are needed. Nevertheless, in the case of rhodium colloids, aggregation of the particles was observed during some catalytic processes, such as in the hydrogenation of arene derivatives,[29] thus justifying the use of extra protective agents. In the literature, various nitrogen donor ligands, such as phenanthroline[30] or polymers like poly(N-vinyl-2-pyrrolidone),[31–33] were used as efficient capping agents of

Alain Roucoux was born in Valenciennes (North of France) in 1963. He received his Ph.D. degree from the University of Lille in 1992 under the supervision of Prof. A. Mortreux in the field of the homogeneous catalytic asymmetric hydrogenation. In 1993, he joined the teaching staff of the Ecole Nationale Superieure de Chimie de Rennes (ENSCR) before becoming Assistant-Professor (Ma^ıtre de Conferences) in 1994. In 2001, he received his Habilitation and was promoted Professor in 2003. Since 1997, he develops the synthesis of noble metal nanoparticles in water for sustainable catalysis. He manages the Nanocatalysis group of the Organometallics: Materials & Catalysis Team (UMR CNRS 6226) – He is the author of about 100 international scientific publications (h527 with one paper in Chem. Rev. 2002 > 1300 times quoted), 5 book chapters and 8 patents.

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Fig. 2. Scientific production dealing with nanospecies in water during the last twenty years extracted from SciFinder database search with the combined research topic “nanoparticle or colloid”, refined with “catalyst” and “water”.

nanocatalysts, to improve the catalyst’s durability and also to potentially tune the selectivity of the catalytic reactions. Similarly, our group has described in 2008 the use of extra bipyridine-based ligands, combined with various ionic liquids, as a relevant “duo” (Figure 4).[34] 2.1. From Bipyridines to Polynitrogen Ligands Catalytically active suspensions of rhodium(0) nanoparticles were easily obtained by chemical reduction of rhodium trichloride in a monophasic THF-ILs media, followed by the consequent addition of the corresponding N-donor protective agent. In this protocol, the ionic liquid and/or the solvent provides a weak interaction on the particle outer shell, supplanted by the stronger stabilization of the N-donor ligand added.[34]

Audrey Denicourt-Nowicki obtained her PhD degree from the University of Lille under the guidance of Dr F. AgbossouNiedercorn and Pr A. Mortreux (2003). After working as a post-doctoral fellow with Pr C. Moody at the University of Exeter (United Kingdom), she joined the Ecole Nationale Superieure de Chimie de Rennes (UMR CNRS 6226, Institut des Sciences Chimiques de Rennes) as a senior lecturer in 2004. Her research interests include the synthesis of metal nanoparticles, either in solution or on support, for applications in polyphasic catalysis. She is author of 54 publications and 5 book chapters.



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Fig. 3. Preparation of metal nanoparticles in water and applications.

Fig. 4. Nanoparticles protected by extra N-donor ligands in various ionic liquids: A pertinent association for polyphasic catalysis.

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Fig. 6. Influence of the stabilizer on the selectivity of the selective styrene hydrogenation in [BMI][PF6] (adapted from ref. [36] and [37]).

Fig. 5. Influence of the ionic liquids (ILs) in selective styrene hydrogenation using 2, 20 -Bipyridine-stabilized Rh(0) nanoparticles (adapted from ref. [34]).

First, the influence of the ionic liquids (ILs), namely the combination of a cationic head and an anion,[34] was studied in the model hydrogenation of styrene (40 bar H2, 808C), using 2,20 -bipyridine-protected Rh(0) nanocatalyst. Catalytic performances were analyzed in terms of the selectivities based on the reduction of the exo C-C double bond vs. the hydrocarbon aromatic cycle (Figure 5). As previously reported in the literature,[35] the coordination ability of the anionic species of the ILs toward the metal surface greatly influences the selectivity. Thus, strong coordinating anions, such as N(CN)2-, lead to the formation of ethylbenzene as sole product with no reduction of the aromatic ring. Likewise, according to the judicious choice of the IL cation, the selectivity in the styrene hydrogenation could also be tuned (ethylbenzene vs. ethylcyclohexane). Second, various N-donor ligands were evaluated, providing an efficient stabilization of Rh(0) colloids in [BMI][PF6],

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even after catalytic reactions (Figure 6).[36–37] According to the chosen skeletons, the selectivity in the styrene hydrogenation towards the sole formation of ethylcyclohexane increases in the following order: TPPZ < 2,20 -Bipy < TPTZ < 3,30 -Bipy 4,40 -Bipy. In the bipyridine range, no significant change in the particle size (2.0–2.5 nm) and organization was observed through TEM analyses (Figure 7). Thus, the differences in kinetic properties and selectivities were explained by the mono- or bi-dentate coordination mode of the protective agent on the particle surface (Figure 7).[36] The 3,30 - and 4,40 bipyridine ligands coordinate to the metal via only one nitrogen atom, allowing an increased flexibility, as well as an easy access of the substrate to the active species and thus a complete reduction of styrene to ethylcyclohexane. In contrast, in the case of the 2,20 regioisomer, the activity decreases owing to a coordination via both N-donor atoms, leading to a more hindered metal surface. This explanation was validated by the selective alkylation of one of the nitrogen atom of 2,20 -bipyridine,[36] affording 2,20 -bipyridinium salt (2,20 -Bipy1) as a monodentate ligand; in that case, the obtained nanocatalyst proved to be more active. To reinforce our hypothesis, similar investigations were carried out on 4,40 -bipyridine, showing no difference in terms of selectivity (100% ethylcyclohexane) in comparison with the nonfunctionalized derivative and thus giving evidence of a mono-coordination mode. Finally, compared to TPTZ (2,4,6tris(2-pyridyl)-s-triazine), the decrease in catalytic activity of the Rh(0) particles coated with TPPZ (tetra-2-pyridinylpyrazine) could be explained by a more coordinating system owing to the additional nitrogen atom.[37] The hydrogenation of various oxygen-containing arenes as model lignin compounds (anisole, cresols) was also investigated, achieving original results with the formation of significant quantities of ketone intermediate.[38]



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2.2. Towards the Use of Functionalized ILs In the last decade, functionalized ionic liquids, also known as task-specific ionic liquids,[39] have emerged as pertinent alternatives, which could alleviate the aggregation phenomenon observed during some catalytic processes and provide nanocatalysts with improved activities and recyclabilities. Among them, ILs based on N-donor ligands, such as pyridine[40–41] or bipyridine,[42–43] were efficiently used as relevant capping agents of metal nanoparticles (Figure 8).

Fig. 7. Influence of the bipyridine regioisomers on the coordination mode and the particle size (adapted from ref. [36]).

In 2009, based on our previous results with bipyridine as an efficient ligand for the stabilization of Rh(0) nanoparticles, our group synthesized a mono-imidazolium functionalized 2,20 - bipyridine ([MIMB][Br]) as capping agent of Rh(0) colloids. Compared to 2,20 -bipyridine, the obtained nanocatalyst showed similar activity in toluene hydrogenation in [BMI][PF6] (Figure 8).[42] Similarly, in 2011, Dyson et al.[43] reported two sterically and electronically distinct bisimidazolium functionalized bipyridines ([BIMB][NTf2]2 and [BIHB][NTf2]2), the second being sterically less demanding and stronger. These bis-functionalized bipyridine ionic liquids, used as capping agents, have a significant influence on the reduction of toluene in [BMI][NTf2]2 (Figure 8). First, the nanocatalyst stabilized by [BIHB][NTf2]2 proved to be significantly more active than the one with [BIMB][NTf2]2, owing to a weaker interaction with the particle surface due to the electron withdrawing effect of the imidazolium cation, as well as the steric hindrance. Moreover, compared to 2,20 -bipyridine, a higher catalytic activity was achieved with [BIHB][NTf2]2 as protective agent, which could mostly be attributed to steric effects. Relying on the outcome of these works, a fundamental study based on various 4,40 -modified bipyridine ligands (with COOMe, Cl, Me, t-Bu, MeO, or NH2 substituents) established the influence of the stabilizer during the catalytic cycle, and more particularly during the ligand dissociation.[44] To strengthen these results, a new capping agent ([dibipy][NTf2]2), based on an electron bipyridine dimer, was designed, affording a relevant catalyst with enhanced

Fig. 8. Towards functionalized bipyridine stabilization. Comparison with 2,2’-bipyridine in toluene hydrogenation (adapted from ref. [42] and [43]).

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Fig. 9. N-(hydroxyalkyl)ammonium salts: a wonderful toolbox for the synthesis of metal nanoparticles in neat water.

properties in terms of stability, activity and durability in comparison with 2,20 -bipyridine-stabilized systems. Nevertheless, a key point concerns the comparative advantages in terms of obtained catalytic properties in regards to the easy access of Ndonor ligands, when comparing the commercial 2,20 -bipyridine with the functionalized bipyridine ionic liquids, which require several synthetic steps.

3. Metal NPs in Aqueous Media: Relevant Tools for Polyphasic Catalysis Water-soluble metal nanoparticles have proved to be suitable catalysts for many organic reactions,[45–47] avoiding the use of harmful organic solvents and allowing easy separation of the catalyst through a biphasic approach.[48–49] In order to avoid aggregation and leaching during the catalytic processes, various water-soluble protective agents (polymers, cyclodextrins, dendrimers, surfactants. . . .), could be efficiently used, affording stable and well-defined particles.[16] Among the amphiphilic compounds, our group designed a library of tailor-made N-(hydroxyalkyl)ammonium salts (HAAX), easily obtained by a scalable and high-yielded synthetic strategy.[50–51] The structural modulations were based on: i) the length of the lipophilic chain, from C10 to C18,[52] ii) the nature of the polar head, with mono- or poly-hydroxyl functions,[15,53–54] to provide enough hydrophilicity, with various carbon chains (C2 to C4),[55–56] and also iii) the counter-ion thanks to anionic metathesis. Without being exhaustive, the structural parameters of this well-equipped toolbox are proposed in the Figure 9.[57–58] The HAAX series (X 5 Br or Cl), with a C2 to C4 polar head, are obtained by reaction of the commercial N,N-dimethylalcoholamine with the appropriate halogenoalkane with good yields,[55,58] while THEA16Cl is prepared by quaternarization of hexadecylamine with chloroethanol.[53]

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Since 2000, more than one hundred compounds have been synthesized and evaluated as protective agents of metal nanospecies according to their modulations. From surface tension measurements, most of these ionic salts display a surfactant behavior, and self-aggregate above the critical micellar concentration (cmc).[52,57–58] The ammonium surfactants were efficiently used as capping agents of various transition metals (Ru, Rh, Pd, Ir, Pt, Au) in water. Notably, in the gold series, these tunable hydroxyalkylammonium salts provide more flexible shape templates than the usual CTAB (cetyltrimethylammonium bromide), giving rise to Au NPs of various morphologies (rods, spheres, prisms. . . .) and sizes, with good yields and selectivities.[55–56] According to the nature of the metal, various catalytic applications in pure biphasic-biphasic conditions (namely, when a liquid substrate constitutes the sole organic phase) were investigated, as reported hereafter. 3.1. Arene Hydrogenation (Rh, Ir, Ru) The hydrogenation of arenes constitutes an important industrial transformation, which finds applications for the production of fine chemicals,[59] and of low-aromatic-content gasoline.[60–61] As well established in the literature,[45,62–64] this reaction is known to be performed using nanoparticlebased catalysts and the examples of purely molecular complexes have been proved to decompose during the catalytic process to form colloidal species.[65–67] Undoubtedly, the first colloidal catalytic system was unwittingly generated in 1983 by Januszkiewicz and Alper,[68] using [RhCl(1,5-hexadiene)]2 for the hydrogenation “in good to excellent yields” of several benzene derivatives with a quaternary ammonium salt as a phase transfer catalyst in a buffer solution. The first complete study was proposed in 1997 by James, who described rhodium and ruthenium colloidal preparation for the hydrogenation of lignin model compounds containing 4-propylphenol fragment in ethanolic solution.[69] In the same time, our group reported



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suspension could be achieved by an increase in hydrogen pressure.[58] Moreover, the surfactant concentration has also an influence on the turnover frequencies in hydrogenation of usual arenes and optimized catalytic activities are reached with a concentration near the critical micellar concentration (cmc).[71] HEA16Cl-protected iridium(0) or ruthenium(0) nanoparticles proved also to be efficient in arene hydrogenations (Figure 11).[72–73] However, compared to their rhodium(0) analogs, higher hydrogen pressure (up to 40 bar H2) is required to achieve efficient catalytic activities. 3.2. Dehalogenation of Halogenoarenes (Pd, Rh)

Fig. 10. Efficient recycling through a biphasic approach. Comparison of various counter-ions of the HEA protective agent (adapted from ref. [58]).

in 1999 its pioneering work concerning the complete hydrogenation of various benzene derivatives by HEA16Cl-protected Rh(0) colloids in pure biphasic liquid-liquid medium (water/ substrate) with recycling under very mild reaction conditions (room temperature and atmospheric hydrogen pressure).[15] Colloidal suspensions of Rh(0) were easily prepared by chemical reduction of rhodium trichloride, with sodium borohydride, in the presence of the ammonium stabilizer. An optimized [Surfactant]/[Rh] molar ratio of 2 proved to be relevant to avoid aggregation of the particles and to achieve significant catalytic activity.[52] In the HEA series, in combination with the hydroxylated polar head, a C16 lipophilic chain provides the most suitable electrosteric stabilization.[15] Moreover, the nature of the counterion has an influence on the morphology of the particles (spherical vs. worm-like), as well as on the catalytic activity in the reduction of arenes (Figure 10).[57–58] 2 In fact, less nucleophilic anions (F2, BF2 4 , and CF3SO3 ) lead to the formation of worm-like particles, owing to weaker interactions within the particle surface. The obtained micellar nanoreactors proved to be pertinent catalysts for the hydrogenation of toluene and anisole under very mild reaction conditions (atmospheric hydrogen pressure, room temperature), and the best performances were achieved with the chloride counterion (Figure 10). The nanocatalysts were also easily reused, with no metal leaching, through an efficient biphasic recycling, the product being extracted with an organic solvent (Figure 10). The use of surfactant-capped Rh(0) colloids was extended to a large variety of arene derivatives (alkylated, functionalized and disubstituted aromatics),[52,57–58] as well as to heteroaromatics.[70] If necessary, an activation of the catalytic

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Hydrodehalogenation reactions, which involve the reductive cleavage of a C-X bond, constitute viable, low-priced and ecoresponsible transformations,[74] and are found to be relevant alternatives to oxidation processes for wastewater treatments.[75] Among the noble metals, palladium has generally appeared as the most suitable candidate to catalyze the liquid phase cleavage of the C-X bond under mild conditions. In that context, HEA16Cl-protected Pd(0) particles were synthesized by the same procedure as their Rh(0) counterparts, based on the reduction of metal salts by sodium borohydride in the presence of HEA16Cl.[76] The obtained palladium(0) nanocatalysts were investigated in the tandem dehalogenationhydrogenation reactions of halogenoarenes, as model substrates of endocrine disruptors. First, as already reported in the literature,[77] bromoarenes present a lower reactivity in the hydrogenolysis process, compared to chlorinated ones, owing to a lower electron affinity of Br which results into a less effective activation of the substrate on the particle surface. Second, the metal nature greatly influences the selectivity of the reaction (Figure 12). In fact, under the reaction conditions (10 bar H2, room temperature), aqueous Pd(0) suspension was only efficient for the cleavage of the carbon-halogen bond, while Rh(0) particles lead to the cyclohexane derivatives, owing to their relevant activity in the reduction of the aromatic ring. The Rh(0) nanocatalysts proved also to be active in the tandem dehalogenation-hydrogenation process of mono- or poly-halogenoanisoles,[78] affording a relevant approach for the detoxification of these hazardous compounds into less harmful products. However, in some cases, particle aggregation could be observed. 3.3. Carbon-Carbon Coupling (Pd) For the last two decades, palladium(0) nanoparticles have proved to be valuable catalysts for various C-C coupling reactions.[79–80] They afford some advantages fitting with the concepts of a sustainable chemistry, even if the true nature of the active species has been extensively discussed. Many reviews have proposed that nanoparticles species potentially act as a



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Fig. 11. Comparison of NPs species based on 3 different metals (Rh, Ir, Ru) in anisole hydrogenation (adapted from ref. [58,72] and [73]).

molar ratio of 25 is needed to improve the stabilization of the metal nanospecies within the aqueous phase under the catalytic conditions. These Pd(0) nanoparticles were evaluated in the Suzuki reaction of various aryl halides or triflate with sodium tetraphenylborate as a relevant water-soluble phenyl-donor.[89] The reactions were performed under mild conditions (608C, 1 equivalent of Cs2CO3), in neat water (Figure 13). However, the reaction was limited to the sole bromoarene derivatives. The surprising result achieved with iodobenzene could be attributed to the formation of aggregates due to iodide poisoning effect, as demonstrated by a poison test with diiode.[48] The reaction was extended to functionalized bromoarenes. 3.4. Asymmetric Hydrogenation Reactions (Rh, Pt) Fig. 12. Dehalogenation reactions of halogenoarenes. Pd vs. Rh nanoparticles (adapted from ref. [76]).

“reservoir” of homogeneous complexes.[81–84] Among traditional C-C coupling such as Heck, Stille and Sonogashira reactions, active nanocatalysts for Suzuki reactions are commonly accepted and the use of non-usual media, such as ionic liquids[85–86] or water,[87–88] has also received a great interest. In that context, our team has developed aqueous suspensions of Pd(0) colloids by chemical reduction of Na2PdCl4 salt in the presence of the HEA16Cl surfactant. A HEA16Cl/Pd

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In the last decade, the use of soluble chirally modified metallic nanoparticles has been reported in the literature for asymmetric catalysis, particularly for hydrogenation processes[90] as well as in bond formation reactions,[91] but still remains ambitious, in particular in water as reaction media. One of the main challenges relies on a good transfer of the chiral information of the capping agent to the substrate near the chirally modified particle surface. First, in 2004, our group developed a catalytic system based on HEA16Cl-capped Pt(0) nanoparticles, modified by cinchonidine, a well-known chiral inducer in heterogeneous asymmetric catalysis.[92] This catalyst was successfully used in



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Fig. 14. Optically active surfactants as protective agents for metal nanoparticles for asymmetric catalysis.

Fig. 13. Sustainable C-C coupling reaction with HEA16Cl-protected Pd(0) nanoparticles (adapted from ref. [89]).

the pure biphasic liquid-liquid asymmetric hydrogenation of ethylpyruvate over several runs, with a constant e.e. value of 55%.[93] More recently, inspired by our approach relying on the electrosteric stabilization of nanospecies by ammonium surfactants, we designed novel water-soluble optically active ammonium salts (Figure 14), based on various polar heads (Nmethylephedrine, N-methylprolinol or cinchona derivatives) in combination with different counterions.[94–95] These quaternary hydroxylated ammonium salts proved to be efficient capping agents for spherical rhodium(0) nanospecies with mean sizes around 0.8 to 2.5 nm. The obtained nanocatalysts were evaluated in the asymmetric hydrogenation of ethylpyruvate in neat water, under optimized reaction conditions (10 bar H2, room temperature). First, the optically active N-methylephedrinium polar head, in combination with a bromide or (S)-lactate counterion, proved to be the most effective in terms of asymmetric induction, affording a modest e.e. value of 12% (Figure 15).[94] Second, the addition of (2)-cinchonidine as an extra optically active inducer, did not relevantly increase the enantiomeric excess (e.e. up to 18%). The use of these nanocatalysts was simultaneously extended to the asymmetric hydrogenation of disubstituted arenes, a more challenging reaction, since Rh(0) colloids are pertinent catalysts for this transformation. Although no asymmetric induction was observed, pertinent catalytic activities were achieved with high cis-selectivities, as usually observed with heterogeneous catalysts.[96–97]

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Low asymmetric inductions observed could be explained by the weak interaction of the ammonium salts within the particle surface, contrary to coordinating ligands, thus allowing the displacement of the optically active inducer by incoming substrates. Finally, our investigations in asymmetric hydrogenations were essentially based on rhodium(0) nanoparticles as active species in the reduction of benzene derivatives and were extrapolated to model substrates (ethylpyruvate, ethylitaconate). Nevertheless, the use of other metals such as platinum, ruthenium or iridium, as well as other prochiral molecules, could have been more relevant to evaluate the ambitious but putative dual role of a unique protective agent to provide efficient stabilization of the metal nanospecies and to generate a pertinent enantiodiscrimination. Our asymmetric approach in water still remains a challenge, even if some interesting results in terms of e.e. values have recently been reported justifying the interest of this research area by academic groups,[98–99] a preliminary step before future industrial applications. 3.5. Towards Other Applications Based on our approach and know-how in the aqueous synthesis of nanoparticles starting from metal salts, the obtained colloidal suspensions have also be used as a “reservoir” of welldefined preformed nanospecies for their phase transfer into a different reaction media, as well as their deposit on inorganic supports. First, in 2005, our team reported an efficient route for the complete and one-step transfer of Rh(0) hydrosol nanoparticles, with mean sizes of 2.4 nm, from water to room temperature ionic liquid [HEA12][NTf2] (where HEA12 5 N, N-dimethyl-N-dodecyl-N-(2-hydroxyethyl) ammonium).[100]



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This phase transfer occurs by anion exchange of HEA12Cl, used as protective agent of nanospecies in water, upon addition of LiNTf2 (Figure 16). The obtained Rh(0)@[HEA12][NTf2]

particles, with diameters around 2.8 nm, were active for the reduction of the exocyclic double bond of styrene, under mild conditions (1 bar H2, room temperature) in [BMI][PF6] and could be easily recycled. Second, the deposit of rhodium(0) nanoparticles on inorganic supports (SiO2 or TiO2) was also developed through a sustainable and easy wet impregnation of the support with the pre-stabilized colloidal suspensions in neat water, without any calcination step (Figure 16).[101–102] Both Rh(0) nanocatalysts, supported either on SiO2 or TiO2 with very low rhodium loading (0.09%), displayed good activities under mild reaction conditions, in neat water, in the hydrogenation of various arene derivatives,[101–102] as well as on bicyclic arenes and heteroaromatics (Figure 17).[103] Higher turnover frequencies up to 30000 h21 were achieved with Rh(0) colloids supported on TiO2, compared to Rh@SiO2 nanomaterial.[103] These catalytic systems could be easily recycled through filtration and reused over several runs, as showed on Figure 18 for Rh@TiO2 nanocomposite. Moreover, they also provide a relevant alternative to colloidal suspensions, in the case of their aggregation under the catalytic reaction conditions, as observed in the tandem hydrodehalogenation reactions.[78] An innovative process based on dry impregnation in a fluidized bed was also developed for the preparation of rhodium composite nanomaterial, by spraying an aqueous colloidal suspension on porous silica particles.[104] This approach enables an easy control of the metal loading and location in the support.

4. Conclusion and Outlook Fig. 15. Chirally modified Rh(0) nanoparticles for asymmetric hydrogenation of ethylpyruvate in neat water. The best anion-cation combination (adapted from ref. [94]).

In the drive towards more eco-responsible chemical processes, our Nanogroup developed efficient and reusable catalysts based

Fig. 16. From colloidal suspensions to a) nanoparticles phase transfer in ionic liquids or b) SiO2- or TiO2-supported nanoparticles.

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Fig. 18. Efficient recycling of Rh@TiO2 in the arene hydrogenation (adapted from ref. [102]).

Fig. 17. Rhodium supported nanoparticles vs. Colloidal suspension – The match in the hydrogenation of arene derivatives (adapted from ref. [78,101] and [102]).

on transition metal nanoparticles for catalytic applications in nonusual reaction media. Our strategy relies on a handful of advantages, fitting well with a sustainable chemistry, such as operational simplicity, use of inexpensive and readily available starting materials and retrievable catalytic systems, mild reaction conditions, associated with clean reaction profiles and easy separation procedures. In one hand, the active species were efficiently stabilized in ionic liquids, thanks to N-donor ligands and showed pertinent activities in the hydrogenation of arene derivatives, as well as an efficient reusability over several runs. On the other hand, a large library of 2-hydroxyethylammonium salts was designed as suitable protective agents of various metal (Rh, Ru, Ir, Pd, Pt, Au. . ..) nanospecies in water, thus affording a wellequipped toolbox for future investigations. According to the metal, various catalytic pure biphasic liquid-liquid applications in mild reaction conditions were investigated, such as hydrogenation, dehalogenation and Suzuki reactions. However, the asymmetric hydrogenation of prochiral substrates in neat water, with optically pure surfactants as capping agents and chiral inducers, proved to be more difficult in terms of enantiodiscrimination. Although many advances were achieved in the last fifteen years concerning the use of soluble nanometer-sized catalysts, some challenges still need to be addressed. These include catalytic reactions which are successfully performed by homogeneous or supported catalysts (C-H activation, asymmetric catalysis or olefin metathesis), but also the rational design of multifunctional and robust nanocomposite catalysts, with

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improved catalytic performances for multiple catalytic reactions with high atom efficiencies, and their industrial applications. Thus, the authors hope that this account will motivate the coming generation of the benefits of soluble metal nanocatalysts, and of the remaining concerns. First, in contrast to the chiral homogeneous complexes, enantioselective nanocatalysis still remains an underexplored area, due to the multiple potential active sites, which render them complex and less successful systems.[14] C-H activation reactions with soluble metal nanoparticles could also be promising in the quest of original reactivities and selectivities, under mild reaction conditions. At the current time, we successfully used ruthenium(0) colloids for the pure biphasic (water/substrate) oxidation reaction of various saturated cyclic hydrocarbons, and particularly cyclohexane, with high selectivities and conversions, thus affording a sustainable and mild route to cyclohexanone production.[105] This preliminary result should open new perspectives for the use of metallic nanospecies in neat water, and their applications for novel sustainable processes. Moreover, in the last years, two accounts outlined the relevance of nanocatalysts for various organic transformations, as well as their opportunities in the synthesis of complex molecules.[106–107] Finally, several axes should be rationalized to guarantee an industrial development, through a judicious design of novel nanocatalysts to optimize efficiencies estimated by TOF (TurnOver Frequency), and catalytic lifetime expressed in TTO (Total Turnover Number). In this context, new strategies have emerged with significant investigations in the field of supported nanoparticles. Currently, the heterogeneization of soluble metal nanocatalysts[108–110] has appeared to be fashionable, being at the meeting point between homogeneous and



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heterogeneous advantages, and offers many possibilities in terms of supports (inorganic matrices, meso- and microporous materials, magnetic supports,. . .).

[20]

Acknowledgements

[21] [22]

The authors thank all co-workers (PhD students and postdoctoral workers) and academic or industrial collaborators for their great contribution in the works reported in this personalized review. Financial support from the Ministe`re de l’Enseignement Superieur et de la Recherche, the Region Bretagne, the Institut Franc¸ais du Petrole (IFPEN) are gratefully appreciated.

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Received: March 21, 2016 Published online: July 18, 2016



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